Arthroscopic impedance probe to detect cartilage degeneration

ABSTRACT

The change in tissue impedance due to the change in the extracellular matrix that results from the degradation of cartilage is utilized to detect degradation of articular cartilage. A probe comprising electrodes is applies a current to the articular cartilage which results in a current distribution and electric field within the cartilage, along with an associated voltage drop across the electrodes. The amplitude of this voltage drop is then measured and divided by the current applied to determine the tissue impedance. By measuring the impedance of patient tissue and comparing the detected patient impedance to a normal value for the tissue from clinically normal tissue, a determination of whether the patient tissue is degraded and the extent of degradation is possible. Preferably, the impedance is measured using a probe with interdigitated electrodes. By changing which electrodes are utilized, the wavelength of the current distribution changes, allowing the probe to image depth dependent focal lesions.

PRIORITY INFORMATION

This application is a divisional application of Ser. NQ. 10/324,717filed Dec. 19, 2002 which is a divisional application of Ser. No.09/776,254 filed Feb. 2, 2001 now U.S. Pat. No. 6,735,468, which claimspriority from provisional application Ser. No. 60/179,820 filed Feb. 2,2000.

SPONSORSHIP INFORMATION

This invention was made with government support under Subcontract No. AR42285, under Prime Grant No. 2 R44 AR42285 02A1, awarded by the NationalInstitutes of Health. The government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

The invention relates to the field of non-destructive arthroscopicdiagnostic probes, and in particular to non-destructive arthroscopicdiagnostic probes for detecting degeneration of articular cartilageutilizing impedance measurements.

Articular Cartilage

The function of organs in the human body are a direct consequence oftheir inherent structure. The function of an organ as a whole is morethan the sum total of its individual constituents. Articular cartilage(AC) is a rich and illustrative example. An understanding of thecomposition and physical properties of AC are essential to diagnose adisease with any given device to aid in patient care. AC is a dynamic,living tissue that responds to stimuli in its environment (i.e. externalloading, fluid flow, electric fields), and the cells of cartilage(chondrocytes) are able to maintain its intricate extracellular matrix(ECM). The scientific data collected over the past 25 years for normalcartilage, supports a hypothesis that a feedback between mechanicalstimulation and chondrocytes must exist to maintain cartilagehomeostasis.

By gross visualization, during knee arthroscopy or an open jointprocedure, normal AC appears as a homogeneous shiny white substancecovering the ends of articulating bones. It is a thin layer from 1 mm to6 mm depending on the joint and particular surface location. In thepresence of synovial fluid, AC provides a very low friction surface thathas a coefficient of friction that is less than that of ice on ice. Acloser inspection at the light microscopic level reveals AC as a verycomplex ECM of macromolecules with chondrocytes embedded within.Cartilage is a unique organ as it is aneural, alymphatic and avascular.Nutrient exchange to the chondrocytes proceeds by diffusion fromsynovial fluid at the articular surface and from the subchondral bonebelow. The lack of a blood supply severely limits AC's ability to berepaired following injury. The absence of innervation means that pain istransduced from the surrounding bone through unshielded force, or fromthe joint capsule in response to an inflammatory stimuli response.

The complex structure of AC acts as a loadbearing, shock absorbing, andwear resistant material to protect joint surfaces. In addition to a lowfriction surface, AC has a high compressive strength critical to painfree joint function. Compressive loads are distributed over a largerarea, while also acting as a damping element during high impact loading(i.e. jumping). Cartilage is also strong in tensile loading when subjectto shear stresses due to the sliding nature of joint function (i.e. kneejoint or intervertebral disk). Lubrication by the synovial fluid alsoreduces shear stresses and helps protect the cartilage from trauma.These macroscopic mechanical properties are a direct consequence of thecomposition.

Articular cartilage is mostly water (60-80% of total weight) and ECMthat comprises the bulk of the dry weight. The primary structuralcomponents of articular cartilage (AC) ECM are produced and maintainedby the chondrocytes enmeshed within it. Tissue mechanical propertiesdepend on the organization and structure of macromolecules present inthe ECM. The ECM is made up of mainly collagen type II fibrils (alongwith small amounts of types IX and XI collagen), charged proteoglycans(PGs), and cells. Collagen and PGs form the framework for cartilage thatresists applied mechanical forces. The collagen forms a dense crosslinked network with PGs embedded within. Proteoglycans aremacromolecules that contain polyanionic sulfated glycosaminoglycan(sGAG) chains. The negative fixed charge density of sGAGs isapproximately 5.3 mEq/gm dry weight in normal human femoral headcartilage. A slight excess of mobile positive ions within the tissuepreserves electroneutrality. At the macroscopic level a Donnan osmoticswelling force develops, caused by the electrostatic charge repulsionbetween the fixed anionic groups that draws water into the ECM,expanding the collagen network.

The chondrocytes are responsible for PG turnover (synthesis anddegradation). The most abundant PG is aggrecan, which has an extendedprotein core with up to 150 chondroitin sulfate and keratan sulfatechains attached in a “bottle brush” structure providing a highconcentration of anions. When first synthesized, aggrecan is mobile, butquickly binds to immobile hyaluranon, stabilized by a link protein,creating the high density of fixed COO⁻ and SO₃ ⁻ groups at physiologicpH.

Many other soluble factors play an important role in the maintenanceprocess by participating as mediators of turnover and production of ECM,including ions, growth factors, hormones, cytokines, proteinases (e.g.matrix metalloproteinases) and their inhibitors. Numerous factors arerequired to maintain homeostasis. They can be produced by thechondrocytes themselves or synthesized elsewhere and transported intothe ECM. These factors affect the chondrocytes through cell surfacereceptors and their transport through the ECM can be prohibited,resulting in pathology.

At the ends of articulating joints, the AC is 3-4 mm thick, with areason the patella as high as 6-8 mm. Microscopically mature AC has 3 zonesbased on the shape of the chondrocytes and distribution of the type IIcollagen. The tangential layer has flat chondrocytes, tangentialcollagen fibril orientation and a sparse PG content. The intermediatelayer is the thickest, with round chondrocytes, oriented in verticalcolumns. Finally, the basal layer has round chondrocytes and containsthe tidemark that separates the uncalcified (nourished by the synovialfluid) and calcified cartilage (that gets fed by the episphysealvessels). It has been reported that no age changes after maturation arediscernible based on histology, including no loss of AC.

Collagen makes up the majority of the dry weight (approximately 50%) ofAC, as it is also the most common structural protein in the body. Incartilage the most abundant form of collagen (>90%) is type II that actsas the structural meshwork of the ECM with its associated extensiveintermolecular crosslinking via trivalent hydroxylysyl pyridinolineresidues. The name “collagen” is a generic term for structural moleculesthat are rich in glycine, proline and hydroxyproline. Striated fibrils,type I, II, and III have three polypeptide chains wound in a triplehelical configuration.

Type II collagen is composed of three left handed tightly interwovenalpha chains (α(II)₁), 300 nm long and 1.5 nm in diameter, each with arepeating amino acid sequence of GlyPro(Hydroxyproline). It is thetriple helical structure that enables collagen type II to have a hightensile strength. Hydroxylysine (some as hydroxylysyl pyridinolinecrosslinks) helps type II collagen link together the ECM network.

Small amounts of collagen type IX help connect the various matrixelements together while type XI (approximately 3%) regulates the caliberof the fiber. In addition, collagen types VI and X are also present(<1%). Collagen type VI has a cross linking behavior and an increasedamount has been reported in OA models, while collagen type X isassociated with growth plate cartilage in the hypertrophic zone, and inthe calcified layer of mature cartilage.

Individual aggrecan monomers are attached to a GAG core (hyaluronate),stabilized by link protein, with the number depending on the functionalnature of the cartilage. The common PGs that can be found in cartilageinclude aggrecan, decorin, fibromodulin, and biglycan and make up about35% of dry weight of AC. Aggrecan molecules form large aggregates(approximately 200 MDa) in cartilage, forming a hydrogel-like structurethat is, in turn, immersed within the collagen type II fibers.

Aggrecan is the major PG in AC (around 90%) and is composed of two typesof sulfated GAG (sGAG): chondroitin-6-sulfate, chondroitin-4-sulfate(approximately 20,000 MW) and keratan sulfate (approximately 5,000 MW).The amount of sGAG attached to each PG varies depending on thefunctionality and integrity of the tissue. Chondroitin sulfate GAGs arechains of repeating disaccharide units that contain highly chargedcarboxylate and sulfate groups. The high density of negatively fixedcharge groups helps attract positive ions and create an osmotic swellingpressure to imbibe water within the tissue. There are approximately100-150 sGAG chains per aggrecan molecule, while extremely largeaggregates can bind thousands of GAG chains.

At the molecular level, the main unit of the aggrecan molecule is aprotein core of approximately 300,000 MW. It has 3 associated globulardomains: G1 and G2 at the N-terminus, and G3 at the C-terminus. The GAGsare mostly contained within the G2 to G3 domain. Keratan sulfate chainsbind, in general, closer to the G1-G2 region (interglobular domain). Theaggrecan PG varies in total size due to the varying amount of boundchondroitin sulfate. Therefore, proteoglycan is a combination of 5%protein and 95% carbohydrate.

Variation in the concentration of PGs has been observed with depth fromthe articular surface in immature and mature tissue, as well as locationwithin a joint. Areas bearing higher stresses have also been shown tohave a higher PG content. The charge groups on the GAG chains areionized at physiologic pH providing a large osmotic swelling pressurethat largely determines the equilibrium compressive modulus.

The main function of chondrocytes involves replenishment ofmacromolecular ECM constituents for its preservation in its harshmechanical environment. With respect to the volume of the cartilage, thechondrocytes account for less than 5% and have a density ofapproximately 20×10³ cells/mm³. In fully developed tissue the size,shape and density of the chondrocytes vary with depth proceeding downfrom the articular surface. In general, the size increases, the shapemoves from flat and elongated to spherical, and the density of cellsdecreases with increasing depth toward the underlying subchondral bone.

In order to perform their biosynthetic functions, chondrocytes are wellequipped with an extensive endoplasmic reticulum and Golgi apparatus aswell as mitochondria and secretory vacuoles. Chondrocytes are alsoinvolved in regulation of ECM assembly and repair by secreting andmediating their assembly. Because of its ability to produce degradatingenzymes as well as their inhibitors, the chondrocytes are believed toparticipate in the physiologic as well as in the pathologic degradationof ECM.

Chondrocytes have also been shown to be able to adapt to the changes bybiosynthetically responding to chemical, physical, mechanical andelectrical stimuli through their cell receptors. These adaptations,through balancing the homeostasis of the ECM, alter integrity to conformwith the stimuli. Biomechanical stimuli of cartilage explants (static orsmall amplitude dynamic compression) have been found to influence therate of aggrecan synthesis and catabolism. This behavior may be due tochanges in cell shape, specific cell-matrix interactions or change theavailability of growth factors.

Investigators are also now beginning to look past the cell membrane toexamine the intracellular changes due to mechanical compression on mRNAlevels and also how cell deformation affects intracellular organelleslike Golgi apparatus or endoplasmic reticulum to fulfill theirfunctions.

Water is the most abundant component in AC, and it appears to becompartmentalized. Water that exists in the interstices of collagenmolecules and fibrils is intrafibrillar water with the balance in theextrafibrillar space. The distribution between these two compartmentshas been reported to be a function of the fixed charge density andloading configuration of the tissue.

The total amount of water present is dependent on the interactionbetween the collagen and sGAG components, as the collagen fibrilreinforcements in the tissue prevent full expansion by the sGAGs (due totheir fixed charge density) and thus constrain water intake. Thisbalance is perturbed in OA cartilage. An increase in water content isobserved compared to healthy cartilage, despite an observable reductionin GAG content. The explanation of this contradiction lies in theassumption that damage to the collagen network severely impairs itsability to restrain the sGAG swelling pressure (despite its lowerconcentration in the tissue), thus the amount of water increases. One ofthe hallmarks of pathologic cartilage is its increased water content.

Matrix metalloproteinases (MMP) are an important group of zinccontaining enzymes responsible for the breakdown of ECM components suchas collagen and PGs in normal embryogenesis and remodeling as well as inmany disease processes like cancer, osteoporosis and arthritis. Theseenzymes are almost universally distributed among mesenchymal cells ofall types, and in some epithelial and endothelial cells as well. Thefamily of MMPs can be divided into 3 subclasses: collagenases (MMP-1,MMP-8, and MMP-13), gelatinases (MMP-2 and MMP-9), and stromelysins.

The collagenases MMP-1 and MMP-13 are members of the family of matrixmetalloproteinases that play an important role in the degradation andturnover of the ECM molecules such as type II collagen and aggrecanduring normal remodeling (e.g. embryogenesis) and disease processing.MMP-1 and MMP-13 have been implicated in the progression ofosteoarthritis and rheumatoid arthritis since they are found in theassociated tissues at higher levels than in normal human tissue. Theactive enzyme has also been found at the lesion sites on the tibialplateaus of Hartley guinea pigs during the progression of spontaneousosteoarthritis. MMP-1 cleaves the three chains of the type II collagenmolecule at specific sites, producing helical trimeric fragments of ¾(from the N-terminus) and ¼ in length that can be detected usingneoepitope antibodies. MMP-13 initially cleaves type II collagen at thissame site, but can additionally cause a second cleavage, residuescarboxy terminal to the primary cleavage site, and then a third cleavagesite another three residues carboxy terminal to the second cleavagesite. The time course of damage induced by MMP-13 is more rapid andtransient than that due to MMP-1; MMP-13 was also found to turn overtype II collagen 10 times faster than MMP-1 in humans.

It has been recently demonstrated through immunostaining of neoepitopesin cartilage from patients with osteoarthritis that type II collagendegradation was initiated by MMP-1 and MMP-13 at the articular surfaceand then extended into the middle and deep zones. In addition to thisdepth dependence, a direct correlation between the Mankin score of OAcartilage degradation and the intensity of the immunostaining of MMPswas recently found.

Electromechanical Effects of Articular Cartilage

As a consequence of the composition of cartilage, measurableelectromechanical properties are exhibited (an area of research referredto as cartilage electromechanics). To consider modeling and analysis ofAC behavior, AC can be conceptualized as a composite material of afibrous mesh (collagen) embedded in a highly hydrated charged gel ofPGs. Resistance to tension and shear loading is provided by thecollagen, whereas the high swelling pressure of the PGs enablescartilage to resist compressive loading. The engineering properties ofcartilage, i.e. compressive and tensile strength, have been assessed inmany different systems. Extraction of PGs has produced a marked decreasein the tissue's equilibrium compressive modulus without effects to thetensile stiffness, while selective degradation of the collagen networkpredominantly affects tensile properties of the tissue.

The dynamic behavior of cartilage is the result of interactions betweenthe solid ECM components and the interstitial fluid. Cartilage is mostsuccessfully modeled as a poroelastic medium. Such a medium is a fluidsaturated porous material in which viscous effects are predominantly dueto frictional interactions between the fluid and solid phases. Theliterature documents a rich history of mathematical models to describethis physical behavior of cartilage. Early work was begun by Biot almostfifty years ago in the context of geophysics. Using a mixture theory,where material properties and constitutive relations are derivedseparately for the fluid and solid phases, a biphasic theory describingthe behavior of cartilage was developed by Mow and coworkers. Thesemodels of fluid flow can be related to mechanical properties likestiffness. Interest today remains high, with continued efforts toproduce more complex nonlinear models for use with today's powerfulcomputational capacity to more realistically model high strain behaviorand the effects of nonhomogeneous material properties on experimentalmeasurements.

Besides having mechanical properties, cartilage exhibits electricalproperties that are coupled with mechanical stresses. It has beenpreviously shown that these electrical interactions play a significantrole in cartilage physiology.

The electromechanical transduction effect is a property of the cartilagecomposition, specifically the PGs and two dual phenomena are observed.

The first of these effects is known as streaming potential. As thehydrated ECM of cartilage is mechanically deformed, a flow ofinterstitial fluid relative to the fixed charge groups of the solidmatrix is created. Entrained positive ions are separated from thenegatively charged matrix macromolecules, giving rise to a voltagegradient or streaming potential in the direction of fluid flow. Thesecond of these effects is the converse electrokinetic effect, known ascurrent generated stress. This effect results when application ofcurrent causes an electrophoretic motion of the negatively charged fixedECM molecules (PGs) towards the positive electrode and an electroosmoticmotion of the mobile ions of the fluid phase towards the negativeelectrode. These combined effects produce a measurable bulk mechanicalstress at the tissue surface that can be detected by an overlying stresssensor.

Osteoarthritis

Osteoarthritis (OA) is the most common disease that directly affects theeveryday mobility and quality of life. It mainly strikes in the lastquarter of life, but also occurs in young people after traumatic sportsinjuries. It attacks the tissue of the synovial joints (e.g., knees,hips, and hands) and is characterized by pain and accompanied bylimitations in joint motion. It can progress to endstage, when patientscan no longer walk pain free. Although the disease itself is not asignificant source of mortality, it is a great cause of physicalsuffering. In the US alone, estimates that the number of peoplesuffering from OA will reach 68 million by the year 2010.

Osteoarthritis describes a group of joint disorders that lead to thedestabilization of normal AC function. Degradation and synthesis bychondrocytes of ECM are uncoupled. The initiation of OA may be a resultof a variety of factors, including genetic, metabolic, developmental,and traumatic. OA is diagnosed clinically with sharp stabbing jointpain, tenderness, and inflammation leading to limitations of jointmovement.

One of the early events in OA at a molecular level is alteration of thecartilage ECM, and the loss of the highly charged macromolecules (PGs)from the matrix. These changes often occur in localized regions ofcartilage along the joint surface and to nonuniform depth. Investigatorshave hypothesized that such molecular changes should change the tissue'smaterial properties.

Traumatic injuries can cause focal defects in cartilage adjacent tootherwise normal cartilage. Clinical repair approaches includedebridement, microfracture, osteochondral plug resurfacing, andchondrocyte transplantation. During surgical procedures, and thesubsequent follow-up, surgeons need to assess the state andfunctionality of the repair tissue. Often remodeling leads to afibrocartilage repair tissue that appears cartilage like but has poorphysical properties that ultimately lead to its failure.

In addition, there is a great need for methods to assess the efficacy oftherapeutic interventions developed to prevent cartilage destruction orthe patency of cartilage repair tissue. Presently, the assessment ofcartilage repair is based on gross and microscopic morphologicalfeatures. Detailed studies have established that the repair tissue isgenerally of good quality in the short term, but fails with time. Atpresent, this behavior is difficult to explain. The literature showsonly a minimal molecular characterization of the types of the cartilagerepair tissue. The more that is understand of the repair process, thehigher the chance to produce the optimal outcome; a repair tissueintegrated in the native cartilage and biomechanically functional formany years. There is also a need for better in vivo, non-destructivediagnostic tools for quantitatively assessing degenerative changes inarticular cartilage and to diagnose OA.

Diagnosis and Monitoring of Osteoarthritis

Current diagnostic criteria and methods for monitoring OA are based onexternal physical examination and x-rays. Efforts are being made todiagnose the disease progression at its earliest stages in order toapply treatment before further damage can occur. Initial diagnosis of OAbegins with patients complaining of pain and stiffness in their joints.Further diagnosis can be made using the current gold standard of x-rayradiography. A grading scheme to characterize the damage is utilized;the most commonly used scale of radiographic evidence is the Kellgranand Lawrence method. The scale is numbered from 0 to 4 with 0 being novisible defects and 4 showing visible OA.

Radiographic diagnostic criteria has three main shortcomings: they lacksensitivity, the emphasis is on changes to the bone, and reading thefilms is subjective with poor reproducibility. Studies have shown thatafter 2 years of treatment with NSAIDS, the radiographs showed nosignificant changes. It is possible that the sensitivity is simply toolow to detect small treatment effects. The limitations of radiographsinclude: nonstandard and shifting of joint positions, the x-ray beamalignment, radiographic magnification not taken into account andlandmarks for measurement can be subject to individual interpretation.These defects have been corrected in current clinical trials. However,x-rays cannot visualize cartilage. They measure the space between bonysurfaces that could be filled with cartilage and thus, are an indirectmeasure of the absence of cartilage or a failure of compressiveresistance of existing cartilage. Thus, x-rays only examine cartilagestatus and can reveal bone and then joint involvement only in the laterstages of disease.

Laboratory methods such as a synovial fluid extraction or a histologicalexamination (Mankin scale) can be used to further investigate theprogression of disease. Unfortunately, synovial fluid extraction canonly rule out other possible causes for the pain, such as rheumatoidarthritis. The Mankin scale categorizes the extent of diseaseprogression in tissue via histology but requires a destructive biopsy.Furthermore, histological examination occurs only where tissue wasremoved.

Other possible methods include MRI, sonography, scintography, andbiochemical markers, but they too have limitations in detecting changesin cartilage. MRI concentrates on biochemical composition studies thatcould yield measures of the sGAG and collagen concentrations, but atthis stage the cost is very high and the fixed charge density too lowfor adequate resolution. As such, clinical MRI approaches do not havethe resolution to show early cartilage changes and do not measurephysical properties.

Arthroscopy has become an important technique in the diagnosis andtherapy of knee OA. A 4 mm diameter arthroscope and/or surgicalinstruments along with a light source is inserted into the jointcapsule, allowing direct visualization of the cartilage surfaces,ligaments, and menisci. The complication rate and morbidity associatedwith the procedure are so low, that arthroscopy is increasingly beingperformed on joints that are only minimally symptomatic as anexploratory procedure. It can detect before degenerative changes areevident by radiography. The recent advent of a 1.8 mm diameter needlearthroscope is transforming arthroscopic examination from ahospital-based procedure to a routine office procedure. During a typicaldiagnostic arthroscopic examination of the knee, the orthopedic surgeoncan inspect the AC surface for gross changes. Arthroscopy is performedvisually without quantitative biophysical methods. Ultimately, thesegross visualization arthroscopic methods, give a visual picture of thecartilage that may or may not correlate with its physical properties.

It is important to note that early OA cartilage may appear normal byvisual inspection. Given that arthroscopy is one of the most commonorthopedic procedures, visual inspection alone during arthroscopy maynot be sufficient for diagnostic purposes, indicating a need forquantitative approaches.

There is currently a commercial arthroscopy blunt probe to subjectivelyassess the degree of softening (known as “grade I chondromalacia”) thatcan result prior to x-ray changes. Another device and method to measurethe physical properties of articular cartilage has been developed byDashefsky (Arthroscopy, 3:80-85, 1987). Dashefsky designed a moreobjective measurement apparatus. He used an instrumented indenterattached to a force transducer to qualitatively assess the mechanicalproperties of chondromalacia of patellar cartilage during arthroscopy.In a group of 107 knees with “patellofemoral symptoms and signs”, 90%were evaluated as “soft;” but over half of these “soft” cartilagesshowed no detectable visual changes of the articular surface of thepatella. Interestingly, of 58 patients with no signs or symptoms of thepatella, 50% showed softening of the cartilage. These results suggestthat physical property changes may not correlate with the patients'symptoms until an irreversible threshold of damage occurs with thechronic wear and tear of cartilage.

More recently, Lyyra et al. (Med. Eng. Phys., 17:395-399, 1995),developed an arthroscopic indenter instrument with strain gauges formeasurement of tissue stiffness in vivo, produced as Artscan 1000 fromArtscan Medical Innovations, Helsinki, Finland. A constant deformationis imposed on the cartilage by the indenter, and the “instantaneous”load response during a one second measurement interval is used toevaluate the tissue stiffness before appreciable stress relaxation hasoccurred. In order to compute an effective dynamic modulus, anindependent measurement of tissue thickness is necessary, as with anyindentation technique. The device was able to detect differences in thestiffness of cartilage in different regions of normal knees.Interestingly, however, the indenter detected only 30-40% decreases incartilage stiffness in the most severely affected regions of thepatellar cartilage of patients with known chondromalacia. Use of thesedevices indicates that purely mechanical tests alone (e.g., indentationtests) may not provide a sufficiently sensitive index of earlydegenerative changes in cartilage. Because of this, the cartilage'selectromechanical transduction properties have been incorporated into asurface diagnostic probe.

This approach has been termed electromechanical surface spectroscopy andutilizes the current generated stress phenomena of articular cartilage.Spatial and temporal changes in the molecular integrity of the collagennetwork due to degeneration lead to important changes in the functionalmechanical and electrical properties of the tissue and therefore causechanges in the current generated stress. In this approach, illustratedin FIG. 1, small sinusoidal electrical currents are imposed by aninterdigitated electrode array 104 that rests on the cartilage articularsurface 100. The current causes an electrophoretic motion of thenegatively charged cartilage extracellular matrix (ECM) towards thepositive electrode and an electroosmotic motion of intratissue fluidtowards the negative electrode. These combined effects producemeasurable normal mechanical stresses at the tissue surface that can bedetected by an overlying piezoelectric stress sensor 102. The stressproduced is at the same fundamental frequency as the driving current,but out of phase due to the poroelastic nature of the cartilageresponse. The penetration depth into the tissue is proportional to thespatial wavelength of interdigitated electrode 104 structure, defined astwice the electrode spacing.

This electromechanical technique is generally described in U.S. Pat. No.5,246,013. In addition, for this technique, Frank et al. (J. Biomech.,20:629-639, 1987) utilized a uniaxial configuration in which the currentwas applied via electrodes on opposite ends of a excised cartilage plug.Sachs et al. later completed a mathematical model showing that twosilver electrodes placed on the same surface side of cartilage couldinduce a measurable mechanical response when current is applied in thispotentially nondestructive arrangement (Physiochem. Hydrodyn.,11:585-614, 1989 and A Mathematical Model of an ElectromechanicallyCoupled Poroelastic Medium. PhD thesis, Massachusetts Institute ofTechnology.). In parallel, Salant et al. (Surface Probe forElectrokinetic Detection of Cartilage Degeneration. MD thesis,Harvard-MIT Division of Health Sciences and Technology.) and laterBerkenblit et al. (Spatial Localization of Cartilage Degradation UsingElectromechanical Surface Spectroscopy With Variable Wavelength andFrequency. PhD thesis, Massachusetts Institute of Technology.) improvedon this design by designing a configuration in which current could beapplied to a single surface of cartilage and the resultant inducedmechanical stress could be measured.

The general technique has also been termed “imposed-k sensing” becausethe medium is excited at a specified temporal (angular) frequency, by anelectrode structure having a spatial period=2/k determined by theelectrode geometry and hence a dominant wave number k. Its advantagesare that it can be made nondestructively (an important requirement forin vivo measurement of cartilage properties) and the electric fieldsgenerated decay exponentially into the material, with a penetrationdepth on the order of /5 to /3. Thus, different depths of the materialmay be tested by varying the imposed spatial wavelength, and spatialinhomogeneities in material properties can be detected by making surfacemeasurements using a series of imposed spatial wavelengths (spatiallocalization). The depth to which the current penetrates into the mediumis proportional to the effective spatial wavelength, which is equal totwice the center-to-center distance between adjacent electrodes. Bychanging the imposed spatial wavelength (by having independentlyaddressable electrodes), various depths of the medium can bepreferentially assessed.

The use of varying wavelengths is illustrated in FIGS. 2 a and 2 b,collectively. Both the frequency and wavelength of the applied currentdensity affect the depth of penetration of the current inducedporoelastic deformation within the tissue. The characteristic depth ofpenetration of the current density, itself, is approximately the spatialwavelength of the current. This wavelength, is determined by theelectrode excitation pattern at the cartilage surface. Therefore, aprobe with four independently addressable electrodes 104 is utilizedsuch that connection to each electrode can be varied externally, therebyenabling multiple wavelengths to be applied using a single device.Applied current densities having short wavelength, illustrated in FIG. 2a, compared to cartilage thickness are confined to the superficialregion of the tissue; the associated current generated stress willtherefore reflect the properties of the superficial zone. In contrast,long wavelength excitations, illustrated in FIG. 2 b, penetrate the fulldepth of the tissue and thereby reflect the average properties of fullthickness cartilage. Thus, combinations of short and long wavelengthexcitations enable the probe to “image” depth dependent focal lesions.

Spectrometer 102 response depends in a sensitive manner on molecularlevel changes in the cartilage matrix similar to changes that occurduring the earliest phases of OA degeneration. These results provide thefundamental basis for the in vivo surface electromechanicalspectroscopic approach to detect cartilage degeneration.

For this method, typically a sinusoidal current density of 1 mA/cm² isapplied to the tissue over the frequency range 0.025-1.0 Hz using abipolar operational amplifier, such that the total current amplitude isconstant at all frequencies, and driven by a programmable frequencygenerator controlled through a computer. The output of the piezoelectricsensor electrodes are passed through a high impedance electrometer, lowpass filtered to remove 60 Hz noise and differentially amplified. Thesignals are recorded on a computer, and combined with a mechanicalsensor calibration done before each test to obtain the current generatedstress. By comparison of this current generated stress to the currentgenerated stress in normal cartilage, a determination can be made as towhether the present cartilage has experienced degradation.

While electromechanical spectroscopy is useful in detecting cartilagedegeneration, there is still an ongoing need for in vivo techniques tonon-destructively and rapidly test for cartilage degeneration.Experiments have shown that use of the electromechanical spectroscopymay be not be able to sensitively measure surface cartilage damage, to adepth of approximately 50 μms, caused by MMP-13. In addition, the mostsensitive detection afforded by current generated stress techniquesrequires measurements at lower frequencies, which require longermeasurement time. The present invention overcomes these disadvantages,in addition to providing further advantages when utilized alone, or inconjunction with electromechanical spectroscopy.

SUMMARY OF THE INVENTION

A probe for detecting the degeneration of mammalian tissue, particularlyarticular cartilage, which comprises a pair of electrodes for applying acurrent to the mammalian tissue is provided. When current is applied tothe tissue there is a distribution of current density and an electricfield within the tissue, and a resulting voltage difference across theelectrodes applying the current. In the preferred embodiment, thevoltage difference is measured by a computer, and the computernormalizes the voltage difference to the current applied to the tissue.This normalized parameter is defined as the tissue impedance. Theimpedance is indicative of the amount of degeneration tissue hasundergone.

In an alternative embodiment, electrodes of the probe comprise aninterdigitated array of electrodes allowing the wavelength, and hence,the depth of penetration of the current distribution into the tissue tobe changed. When a short wavelength mode is utilized, the impedancemeasurements are representative of a superficial layer's degeneration.When a long wavelength mode is utilized, the impedance measurements arerepresentative of the bulk tissue degeneration.

In an alternative embodiment, the probe additionally comprises a stresssensor. The current applied to the tissue creates current generatedstress in the tissue. This stress is measured and additionally used todetermine whether degeneration has occurred. In one embodiment,impedance measurements are taken simultaneously with the currentgenerated stress measurements at the same frequency as the currentgenerated stress measurements. In a different embodiment, the impedancemeasurements are taken sequentially, i.e. after the current generatedstress measurements, at a frequency higher than those at which thecurrent generated stress measurements are made.

A method for detecting the degeneration of mammalian tissue,particularly cartilage, is provided. Current is applied to the tissuecreating a distribution of current density and an electric field withinthe tissue, and a resulting voltage difference across the electrodesapplying the current. The amplitude of the voltage difference across theelectrodes is measured and divided by the current applied by theelectrodes to give the tissue impedance. This tissue impedance iscompared to an impedance value of clinically normal tissue to determineif the tissue is degenerated.

In one embodiment, the voltage difference across the electrodes applyingcurrent is measured simultaneously to a current generated stressmeasurement and at the same frequency as the current generated stressmeasurement. In another embodiment, the voltage difference is measuredat a higher frequency than that used for current generated stressmeasurements, either alone, or after current generated stressmeasurements have been made.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the sensor and electrodes for current generatedstress measurements.

FIG. 2 illustrates the short and long wavelength modes for currentgenerated stress measurements.

FIG. 3 illustrates a system according to the present invention formeasuring the impedance of cartilage to detect degradation of thecartilage.

FIG. 4 shows a schematic of a preferred embodiment of the currentsource.

FIG. 5 illustrates the general method for determining whether cartilagedegradation has occurred in test cartilage utilizing impedancemeasurements.

FIGS. 6 a and 6 b, collectively, illustrate the layers that comprise theelectrode/transducer structure for the preferred embodiment of theprobe.

FIGS. 7 a and 7 b illustrate the electrode patterns for the excitationelectrodes and sensor electrodes of the preferred embodiment of theprobe.

FIG. 8 illustrates the backing plate on the electrode/transducerstructure of the preferred embodiment of the probe.

FIGS. 9 a-9 d, collectively, illustrate the probe structure for thepreferred embodiment of the probe.

DETAILED DESCRIPTION OF THE INVENTION Operation of the Invention

When current is applied to articular cartilage there is a distributionof current density and an electric field within the tissue, and also anassociated voltage drop across the electrodes applying the current. Theamplitude of the measured voltage drop between electrodes divided by theapplied current amplitude is defined as the electrical impedance. Theelectrical impedance of the tissue for frequency ranges at least up toapproximately 1 kHz, and possibly further, is dominated by theresistance of the hydrated ECM and this resistance is inverselyproportional to the density of mobile ions within the intratissue fluid.Thus, the electrical impedance of the tissue will increase withdecreasing PG fixed charge content or increased swelling at constant PGcontent, since both these conditions lead to a lower concentration ofmobile ions in the ECM by Donnan equilibrium. In addition, collagennetwork degradation will alter impedance.

This change in impedance due to the change in the cellular matrix thatresults from the degradation of the cartilage is utilized to detectdegradation of articular cartilage. By measuring the impedance ofpatient tissue and comparing the detected patient impedance to a normalvalue for the tissue from clinically normal tissue, a determination ofwhether the patient tissue is degraded and the extent of degradation ispossible. Preferably, the impedance is measured using a probe withinterdigitated electrodes similar to that used for electromechanicalspectroscopy. In this manner, the probe is able to “image” depthdependent focal lesions.

The use of impedance to determine cartilage degradation provides for anumber of advantages:

-   -   Higher frequencies (e.g. 1 kHz) than those used for        electromechanical spectroscopy can be used, increasing the speed        with which measurements are performed.    -   No need for complex signal processing.    -   For an impedance-only probe, there is no need for a mechanical        calibration step and eliminates the difficulties of        manufacturing probes with the sensitive behavior of a        piezoelectric sensor.    -   When impedance is utilized with current generated stress (CGS,        i.e. electromechanical spectroscopy) measurements (in the low        frequency range, 0.25-1.0 Hz, where CGS is detectable),        impedance is measured simply by monitoring the voltage across        the probe inputs.    -   Electrodes for impedance measurements are able to be made        arbitrarily small in any arrangement to control spatial        sensitivity of the impedance measurements to possible diagnose        cartilage abnormalities in small animals (e.g. rats) where the        cartilage may only be 100 μms thick.    -   Acquisition of data over a wider frequency range.    -   An increased precision of measurements resulting from smaller        standard deviations.

FIG. 3 illustrates a system according to the present invention formeasuring the impedance of cartilage to detect degradation of thecartilage. A waveform generator 300 applies a sinusoidal voltagewaveform to a current source 304 such that the total current amplitudeis constant at all frequencies. A computer 302 controls waveformgenerator 300. The current generated by current source 304 is applied tothe electrodes 310, 312, 314 and 316. Preferably, multiple independentlyaddressable electrodes are utilized so as to be able to generate bothlong wavelength and short wavelength excitations. As previouslydescribed, applied current densities having short wavelength compared tocartilage thickness are confined to the superficial region of thetissue. Therefore, the associated measured impedance reflects theproperties of the superficial zone. In contrast, long wavelengthexcitations penetrate the full depth of the tissue and as such, themeasured impedance associated with long wavelength excitations reflectsthe average properties of full thickness cartilage. Thus, combinationsof short and long wavelength excitations enable the probe to “image”depth dependent focal lesions.

To measure the impedance, computer 302 measures the current applied tocartilage 108 through electrodes 310, 312, 314 and 316. In addition,computer 302 measures the voltage drop across the electrodes applyingthe current to cartilage 308. By normalizing the voltage drop to thecurrent applied, the impedance of cartilage 308 is measured. Theimpedance is then compared to clinically normal tissue to determinewhether degradation has occurred in the tested tissue. It should benoted that, while a computer is preferred to measure the voltage anddetermine the impedance, other equivalent devices such as a voltmeter,an application specific integrated circuit, or a chip internal to theprobe can be utilized to either measure the voltage or determine theimpedance without departing from the scope of the present invention.

Detecting cartilage degradation using impedance relies on detectingchanges in tissue conductivity resulting from loss of aggrecan chargegroups and/or degradation of the collagen network. However, evencomplete loss of GAG from the tissue would decrease the electricalconductivity by only about 28 %, while electrokinetic coupling, and thusthe current generated stress amplitude, would decrease to zero. Thus, asis the case with purely mechanical measurements of cartilage materialproperties, measures of purely electrical properties, such as impedance,may be a less sensitive indicator of degradative changes thanmeasurement of electromechanical quantities such as current generatedstress. Therefore, in the preferred embodiment, the impedancemeasurements are utilized in conjunction with the current generatedstress measurements previously described. When measured in conjunctionwith the current generated stress measurements, impedance measurementsare made in the low frequency range, 0.25-1.0 Hz, where currentgenerated stress is detectable, simultaneously with the currentgenerated stress measurement. However, the impedance measurements areable to be made alone, or following the current generated stressmeasurements. When the impedance measurements are made alone, orfollowing the current generated stress measurements, rather thansimultaneous with the current generated stress measurements, it ispreferable to make the impedance measurements in a frequency range ofapproximately 1 kHz in order to allow the measurements to be takenrapidly, making it less sensitive to surgeon hand tremor. In addition,measurement of cartilage impedance over a range of frequencies couldalso provide additional information about the tissue, such as anestimate of cartilage thickness.

FIG. 4 shows a schematic of a preferred embodiment of current source 104that delivers current to the probe at the cartilage surface. Currentsource 304 is depicted used with a two-electrode probe for simplicity.

Current source 304 comprises a bipolar operational amplifier 400,resistor 402, variable resistor 404 and resistor 406. When a sinusoidalinput voltage (V_(in)) is provided to current source 304 from waveformgenerator 300, a resultant current I_(c) to the cartilage is produced.The current applied to cartilage also results in a voltage differenceV_(out)-V_(m), between the electrodes on cartilage 408. This voltagedifference is measured and divided by the current IC to obtain themeasured impedance Z_(meas).

FIG. 5 illustrates the general technique for determining whethercartilage degradation has occurred in test cartilage utilizing impedancemeasurements. A sinusoidal current is applied to the electrodes on thesurface of the cartilage by a current source 500. As the current isapplied to the cartilage, the voltage difference present across theelectrodes is measured 502 by a computer. The measured voltagedifference is then divided by the current applied to the electrodes toobtain the impedance 504. This impedance is then compared to theimpedance of clinically normal cartilage to determine if degradation ispresent 506.

Exemplary Embodiment of Arthroscopic Probe for Detecting CartilageDegradation

Electrode/Transducer Structure and Fabrication

It should be noted, as the preferred embodiment of the present inventioncomprises impedance measurements utilized along with current generatedstress measurements, the preferred embodiment of the electrodes andprobe include a stress sensor and the following description will includethe stress sensor as part of the electrodes and probe. However, thepresent invention should not be seen as limited thereto.

The electrode/transducer structure (ETS) is the working component of the“probe” system, applying current to the cartilage surface and measuringthe resultant stress from a piezoelectric film. Preferably, it is aflexible, 180 μm thick, 3 layer laminated structure. FIGS. 6 a and 6 b,collectively, illustrate the layers it is comprised of:

-   -   Silver excitation electrodes are etched from a sheet of silver        foil 604 and deposited with a layer of silver chloride in an        appropriate electrochemical cell. Preferably, silver foil layer        604 for the excitation electrodes is made from a 25.4 μm thick        silver foil cut to a size of 18×18 mm with a sharp straight        edge. The use of Ag/AgCl electrodes decreases the low frequency        impedance between the electrode and cartilage while stabilizing        the electrode potential.    -   A shielding layer 602 is preferably made from 25.4 μm thick        Mylar™ polyester film metallized (on one surface) with a thin        layer of aluminum and is cut to 15×15 mm. Shielding layer 602        separates the silver electrodes and stress sensors while also        serving as a ground/isolation plane. The metallization is a        ground plane that helps shield the sensitive stress sensors from        electromagnetic interference (principally electric fields        emanating from the excitation electrodes) when the ETS is placed        in the probe body.    -   The stress sensor is fabricated from a single sheet of 52 μm        thick polyvinylidene fluoride (PVDF) with a thin (100 Å)        sputtered nickel-copper alloy metallization on both sides 600,        known as Kynar™, from AMP Inc., Norristown, Pa. Preferably,        Kynar™ layer 600 is punched to a disk of 4.5 mm in diameter (the        size of the “active” area of the probe). This piezoelectric        material transduces the mechanical stress sensed to a measurable        voltage signal. The metallization on one surface of the Kynar™        is etched to form electrodes that register with the silver        electrodes on the opposite side of the ETS. The other side of        the PVDF is connected to the ground/isolation layer 602. Kynar        is preferable for the stress sensor in this application for a        number of reasons:        -   it has a high sensitivity to mechanical stress especially in            this low frequency (0.025-1.0 Hz) application, allowing the            film to behave as a compact strain gauge with no external            power source,        -   the generated dynamic signals are greater than those from            typical strain gauges after amplification, and        -   the flexible sheets are relatively inexpensive and can be            cut into arbitrary patterns.

To characterize the sensor as a stress gauge, a force applied normal(forces in the plane of the electrode are neglected) to the surface ofthe film develops an electric surface charge on the metallizationproportional to the mechanical stress. The charge Q [coulombs] developedby a stress [N/m²] over an area A [m²] can be described by Q=d_(t)A,where d_(t) is an empirically determined piezoelectric strain constant.The open circuit voltage between the metallization on either side ofsheet 600 is the charge divided by the capacitance, A′/, where is thedielectric constant of the film and is the film thickness. If part ofthe total metallized area is not being loaded, it adds to thecapacitance without generating any charge, thus decreasing the measuredvoltage. Representing the total area by A′ and the active area beingloaded by A, the equation for the open circuit voltage signal becomes:V=Q/C_(total)=d_(t)A/A′/=d_(t)A/A′

For maximum sensitivity, it is desirable to maximize the measuredvoltage signal, V, for a given stress, thus A/A′˜1. The voltage outputalso depends on film thickness. As electrodes are more tightly packedonto the ETS, a thinner film may be needed to gain sufficient spatialresolution with respect to electrode spacing.

The assembly procedure of fabricating this laminated structure ischaracterized in the following phases:

Phase I-ETS Construction

The sheets are rinsed with a mild detergent and then deionized water,while handling is done with disposable latex gloves to keep allmaterials clean to prevent contamination with oils and dirt. To aid inthe photofabrication, both sides of the silver foil are gently abradedwith a fine abrasive and then dipped in a 15% v/v nitric acid solution.To form a laminated structure, silver foil 604 is bonded to thenon-metallized side of the shielding layer 602 using a two-part urethaneepoxy in a 50:1 ratio (e.g. Tycel 7000/7200, Lord Corp., Erie, Pa.)thinned with methyl ethyl ketone. Silver foil 604 is larger to allowpress fit connections with copper tabs in the periphery of the innercore when the probe is assembled. Sensor layer 600 is bonded to themetallized side of the shielding layer 602 with a manually applied thinfilm of silver conducting epoxy (e.g. TRA-DUCT 2902, TRA-CON Inc.,Medford Ma.). The ETS is pressed together for a few minutes to assuregood bonding and is allowed to cure overnight.

Phase II-Photofabrication

To form the silver excitation and the piezoelectric stress sensorelectrodes, standard photofabrication techniques are used. The surfacesare coated with a light sensitive organic polymer, photoresist, whichbecomes inert to the etching chemicals when cross linked by ultravioletlight. A negative of the electrode pattern desired is used toselectively cross link the photoresist, then etching chemicals are usedto isolate the electrodes.

The ETS is dehydration baked at 80° C. for 10 minutes in a convectionoven to remove residual moisture, and both sides are coated with aphotoresist compound, hung to dry for 30 minutes in a darkroom, and thenbaked between paper and glass plates (to keep them flat) at 80° C. foranother 10 minutes. Electrode patterns are converted to negative images(masks) on two photographic transparencies. A dry, photoresist coatedETS is placed between the two masks, aligned so the electrodes areregistered on opposite sides of the ETS, then exposed to ultravioletlight for 15 minutes. The ETS is then bathed in a xylene-based developersolution for 30 seconds, transferred to another bath of developer for 30seconds, and then rinsed under warm tap water and blotted dry. Thedeveloper removes the uncross linked photoresist, leaving the resistbehind in the desired electrode pattern. The electrode pattern for thesensor electrodes on Kynar™ layer 600 is illustrated in FIG. 7 a, whilethe excitation electrode pattern for silver layer 604 is illustrated inFIG. 7 b.

Phase III-Etching

Etching of the silver metallization occurs while the ETS is mounted in acustom made two-part poly(methyl methacrylate) holder with a rubberO-ring gasket to contain the etchant. An appropriate etchant is a 55%w/v solution of ferric nitrate heated to 45° C., with the silver side ofthe ETS exposed to a fresh etchant bath every two minutes. The thinmetallization on the piezo film is etched, by carefully placing a fewdrops of etchant on the surface, waiting only 5-10 seconds, and quicklyrinsing with deionized water. The photoresist is finally removed fromboth sides with a cotton swab dipped in xylenes.

The next step in ETS fabrication involves cutting the ETS into a patternthat enables it to be fit onto the head of the probe. During thephotofabrication step, the outlines of the border were also marked ontothe ETS. Cutting is performed along the borders with a sharp scalpel. Asillustrated in FIG. 8, after cutting, a 0.33 mm thick crucifix shapedplastic backing plate 800 is attached to the piezo side of ETS 802 by atwo-part epoxy and dried overnight. Backing plate 802 helps with thealignment of the ETS, described below, and ensures ETS is flat. Thepresent design consisting of the backing plate and the smaller Kynardisk means that the ETS does not have to formed into its final threedimensional shape with a die. The final active area of the ETS is aflat, wrinkle-free surface against the top surface of the core, with nosmall fold or wrinkle introducing large local stress concentrations thatare sensed by the piezo film, significantly distorting the measuredsignal.

As a final step, a layer of silver chloride is layered onto the silverexcitation electrodes. The fully assembled probe is suspended in a bathof unbuffered 0.1 M NaCl, titrated to pH 4.0 with 1 N HCl, and thepositive terminal of a variable DC power supply connected to one of thesilver electrode wires, in series with an ammeter and a 47 kW resistor.The negative terminal is connected to a platinum strip and suspended inthe electrolyte. A current of 120 μA is run for 10 minutes,corresponding to a total chloride deposition of 1000 (mA-seconds)/cm²,for each electrode of 1.59 mm² which is acceptable for bioelectricapplications.

Probe Structure and Assembly

FIGS. 9 a-9 d illustrate, collectively, the structure of the probe.Generally, ETS 912 is held in place by pressing it against the inside ofa sheath 916 by an inner core 902. This assembly is held inside a shell918 with a screwed pusher/plunger 906.

The first part of the inner core 902 is preferably a stainless steelhead, to accept the stress sensor contacts 910 and is conducting toprovide part of the required shielding. Contacts to the stress sensor onETS 912 are formed by metallic rods 910, preferably brass, at 90°intervals, potted into a recess in the stainless head with anon-conducting two-part epoxy 908. A cruciform pattern is machined inthe hardened epoxy 908 to accept a backing plate 800 constructed on ETS912, illustrated in FIG. 9 d. Contacts 910 are electrically isolatedfrom each other and the ground plane (stainless steel head). Prior topotting, each of the 4 contacts 910 are carefully soldered to a wire acable, which leads to the peripheral circuitry. The cable accommodatesfour coaxial cables that are especially tailored for low noiseapplications. The second part of inner core 904 is a plastic body, thatprovides a means of making the electrical connections to the excitationelectrodes, through 4 slots on the periphery that each hold a copper tab914 connected to a thin wire to carry the driving current.

The insulating (non-conducting) sheath 916 is a thin cylindrical shellmade of plastic. Sheath 916 is fitted over ETS 912 that is placed overthe end of the inner core 914. The end of sheath 916 is open, exposingthe surface of ETS 912 to the cartilage during measurement. Sheath 912is long enough to cover copper tabs 914 to prevent its contact withstainless steel outer body 918, thus isolating the driving current fromground. In addition, the edge of the sheath is angled to press fit ETS912 over the rim of the stainless steel head 902 of the inner core 900.The contacts between the silver electrode arms of ETS 912 and coppertabs 914 of inner core 900 are also stabilized by the sheath 916 andsheath 916 provides the pressure to make contact between the electrodearms of ETS 912 and copper tabs 914. A bead of silicon adhesive to sealthe probe against the aqueous environment is placed around the peripheryof the angled edge before final assembly.

The outer body stainless steel tube 918 is a cylindrical stainless steeltube that acts as a stiff cover to protect the inner components of theprobe. One end of outer body 918 is open to expose the surface of ETS912 but angled to catch the edge on the end of the probe. The other endof outer body 918 is flared outward. Outer body 918 is slid over thesheath/inner core 916. A nut is then slipped over outer body 918, makingcontact with the threads on pusher/plunger 906, which holds the innercore and non-conducting sheath assembly in outer shell 918. The nut alsopulls down upon the flared end of the outer body. As the nut is screwed,outer body 918 is tightened over sheath/inner core 916.

As previously described, the head of the probe has a machined recess 920in the shape of the crucifix so that ETS 912 fits with the properorientation to line up the electrical contacts. Brass contacts 910 atthe head of the probe receive signals from the piezo electrodes whilecopper tabs 914 on the side inner core 900 connect with the arms of thesilver electrodes. The current is driven through wires leading up tocopper tabs 914 and onto the silver electrodes while the currentgenerated stress is transferred to the piezo electrodes and transmittedthrough brass contacts 910 to the output wires. When assembling theprobe, once contacts have been made and pusher/plunger 906 is fittedwith inner core 900, sheath 916 is fitted over the head of the probe,making sure ETS 912 is lying flat on the surface of the head. Beforesheath 916 is completely fitted over the head, the silver leads areslipped under the copper tabs, and the electrical connections to thesilver electrodes are tested. Once all contacts are established, a thinlayer of waterproof silicon rubber adhesive is placed on the inside edgeof sheath 916. Finally the outer stainless steel cylindrical body 918 isslipped over the probe. Adhesive is also placed on the inside edge ofouter body 918 before the probe is finally assembled. Excess adhesive iswiped off the edges. The sealed probe is allowed to dry for at least 24hours. The active surface of ETS 912 extends slightly beyond the tubeend and makes unobstructed contact with the cartilage surface.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. An arthroscopic probe for applying a current to mammalian cartilagein order to determine an impedance of said mammalian cartilage so as todetect degeneration of said cartilage, said probe comprising: an innercore, said inner core having a first surface, a second surfacesubstantially parallel to said first surface and at least one sidesurface connecting said first and second surface; said first surfacehaving a set of electrodes mounted thereon, said electrodes having atleast two electrical conductors extending from said set of electrodesalong said at least one side surface; said at least one side surfacehaving at least two electrically conductive plates thereon, saidelectrically conductive plates for respectively contacting said at leasttwo conductors extending along said side surface, each of said at leasttwo electrically conductive plates having electrical conductorsconnected thereto for providing said current to said electrodes, saidelectrical conductors connected to said at least two electricallyconductive plates extending through a substantially hollow portion ofsaid inner core; said second surface having a recessed portion, saidrecessed portion connecting to said substantially hollow portion of saidinner core; a hollow pusher/plunger having one end thereof engaging saidrecess formed on said second surface, said electrical conductorsconnected to said conductive plates extending from said recessed portioninto said hollow pusher/plunger and extending inside said hollowpusher/plunger; a first sheath covering at least a portion of said innercore, said first sheath having an opening at an end co-located to saidfirst surface for exposing said set of electrodes, and a second sheathcovering said first sheath, said inner core and at least covering aportion of said hollow pusher/plunger, said second sheath having anopening at an end co-located to said first surface for exposing said setof electrodes.
 2. The arthroscopic probe for applying a current tomammalian cartilage in order to determine an impedance of said mammaliancartilage so as to detect degeneration of said cartilage, as per claim1, wherein a first portion of said inner core, said hollowpusher/plunger and said second sheath comprise a conductive material,and a second portion of said inner core and said first sheath comprise anon-conductive material.
 3. The arthroscopic probe for applying acurrent to mammalian cartilage in order to determine an impedance ofsaid mammalian cartilage so as to detect degeneration of said cartilage,as per claim 1, wherein said first sheath provides pressure to said atleast two electrical conductors extending from said set of electrodessuch that said at least two electrical conductors contacts saidelectrically conductive plates.
 4. The arthroscopic probe for applying acurrent to mammalian cartilage in order to determine an impedance ofsaid mammalian cartilage so as to detect degeneration of said cartilage,as per claim 1, wherein said set of electrodes comprises at least fourinterdigitated electrodes, said at least two electrical conductorsextending from said electrodes comprises at least four electricalconductors and said at least two electrically conductive platescomprises at least four electrically conductive plates.
 5. Thearthroscopic probe for applying a current to mammalian cartilage inorder to determine an impedance of said mammalian cartilage so as todetect degeneration of said cartilage, as per claim 1, wherein saidfirst surface comprises a shaped recess formed thereon and said set ofelectrodes comprises a correspondingly shaped backing plate formedthereon, said shaped recess and said shaped backing plate providing aproper orientation such that said electrical conductors extending fromsaid set of electrodes are aligned with said electrically conductiveplates when said set of electrodes is mounted on said first surface. 6.The arthroscopic probe for applying a current to mammalian cartilage inorder to determine an impedance of said mammalian cartilage so as todetect degeneration of said cartilage, as per claim 1, wherein said setof electrodes is a combined electrode and transducer structure.
 7. Thearthroscopic probe for applying a current to mammalian cartilage inorder to determine an impedance of said mammalian cartilage so as todetect degeneration of said cartilage, as per claim 6, wherein saidcombined electrode and transducer structure comprises: interdigitatedexcitation electrodes formed of a conductive material for application ofsaid current to said mammalian cartilage; an insulating sheet havingsaid excitation electrodes bonded to the lower insulating surfacethereof and having its upper surface metalized for electrical grounding,and a piezoelectric polymeric film for transducing mechanical stress toone of a current or a voltage having its upper and lower surfacesmetalized, said lower metalized surface of said piezoelectric polymericfilm bonded to said upper metalized surface of said insulating sheet,said upper metalized surface of said piezoelectric polymeric film formedinto transducer electrodes for transmitting said current or voltage to adetector.
 8. The arthroscopic probe for applying a current to mammaliancartilage in order to determine an impedance of said mammalian cartilageso as to detect degeneration of said cartilage, as per claim 6, whereinsaid probe is additionally utilized to make a current generated stressmeasurement of said mammalian cartilage.